[Oratam]

See that "O" at the beginning of the word? That letter makes the sound "Oh!"...like Omaha, Oklahoma, Oldsmobile.

Ah, right, like the first 'o' in 'osmosis' but not the second. Got it.

[meadsjn]

Like the first “o” in opossum, which is pronounced “Possum”.

[ROCKLOBSTER]

Ah, right, like the first 'o' in 'osmosis' but not the second. Got it

Well, not quite.
There are spelling and pronunciation rules in our language. In the case of 'osmosis'...the first "O" is followed by two consonants, which indicates the "short-O" (ah) vowel. The second "O" is followed by only one consonant which wants the "long-O" (oh) vowel.

At least that's how Mercun English worked 60-some years ago when I was in grammar school.